1. Field of Invention
The invention relates to an image acquisition system for reading information on the top faces of objects being displaced on a conveyor, the system comprising a high resolution camera mounted in a fixed position above the plane of the conveyor to read the top face of each object, said camera being fixed and having a lens system with a motor-driven focusing mechanism.
2. Description of the Related Art
Such a system is already known from document EP-0647479. It is intended more particularly for automatically reading the destination addresses of postal packets or parcels by performing optical character recognition on the basis of the images and for the purpose of performing automatic postal sorting. In that system, an ultrasound pickup determines the height of the plane corresponding to the top surface of each parcel or packet relative to the plane of the conveyor so as to control the motor of the focusing mechanism when the camera begins to observe the top surface of the object.
In that known system, the camera is of the type having photosensitive elements of the charge-coupled device (CCD) type and its depth of field (for a fixed distance between the array of photosensitive elements and the object being observed) depends on two parameters: the magnification factor and the aperture of the lens system. In practice, the magnification factor (ratio of the dimensions between the observed object and its image) is around 9 to 13 for objects such as postal parcels or packets. Also, the aperture of the lens system is limited, given the speed at which postal parcels or packets are conveyed in sorting equipment (about 1 meter per second (m/s) to 2 m/s) and the light energy required for illuminating the parcels or packets while an image is being acquired (2 kW is a maximum). An aperture of 2 seems to be a limit that is difficult to improve, and consequently the maximum depth of field that can be obtained is of the order of 2 cm to 3 cm.
With those constraints, that image acquisition system cannot operate properly when some or all of the parcels or packets have top faces presenting large surface irregularities (corrugations, steps) giving rise to variations in the height of the top face of any one parcel or packet that are significantly greater than the depth of field of the lens system, consequently making it impossible to obtain a sharp image of the information. Also, it is not possible to envisage processing articles of mail disposed in an inclined manner on the plane of the conveyor. Unfortunately, this practice is commonplace since it serves to keep bulky articles of mail such as parcels or packets in a fixed position on a conveyor belt when the belt is moved at high speed (of the order of 1 m/s to 2 m/s).